I could make some of the hollow parts solid, but you would no longer be able to install electronics and what not.
As for foamcore, I could probably use it in the barrel assembly, and the stock, if I were to make it solid, but the main body if the gun is U shaped with lots of cavities.
Even if the entire model was static, it would still be a costly project. The Pulse rifle has a lot of parts hanging off the main body of the gun, such as the heat shields, heat sinks, the magazine and it's assemblies in particular, greeblies (albeit big ones) etc.
So if it were to become static, or quasi static, would $375-400 be more reasonable?